Default CSS used

Basically, if you choose a theme with a variation and then switch to a
completly different theme it tries to use the same variation with that
theme, even if it doesn't exist.

This PR always sets CSS to default when switching theme. Tested and
working fine on my server.

As discussed at http://forums.sentora.io/showthread.php?tid=89
Merge pull request #32 from jacobg830/bug/theme

Reset's the default theme variation each time a theme is changed to ensure that the variation is not re-used on a theme that does not have a variation with the same name.